填表 is a separable verb (离合词) meaning 'to fill in/out a form'. The two characters 填 and 表 can split when you insert a measure word, complement, or object between them — for example, 填一份表 ('fill in one form'), 填好了表 ('finished filling the form'), or 填了表 ('filled in the form'). You cannot put an object after 表 — the form itself is already 表.